Garmin GPS 25 LP Technical Specification

Garmin GPS 25 LP Technical Specification Manual pdf 20 page image

16Section 4Software InterfaceThe GPS 25LP interface protocol design on TXD1/RXD1 is based on the NationalMarine Electronics Association’s NMEA 0183 ASCII interface specification, which isfully defined in “NMEA 0183, Version 2.0” (copies may be obtained from NMEA, P.O.Box 50040, Mobile, AL, 36605, U.S.A.) and the Radio Technical Commission forMaritime Services’ “RTCM Recommended Standards For Differential Navstar GPSService, Version 2.1, RTCM Special Committee No. 104” (copies may be obtained fromRTCM, P.O. Box 19087, Washington, D.C., 20036, U.S.A.). The GPS 25LP interfaceprotocol, in addition to transmitting navigation information as defined by NMEA 0183,transmits additional information using the convention of GARMIN proprietarysentences.Binary phase data information is output on TXD2, see Appendix D for details.The following sections describe the NMEA data format of each sentence transmittedand received by the GPS 25LP sensor board. The baud rate selection, one-pulse-per-second output interfaces and RTCM differential GPS input are also described.4.1 NMEA Received sentencesThe subsequent paragraphs define the sentences which can be received on RXD1 bythe GPS 25LP sensor boards. Null fields in the configuration sentence indicate nochange in the particular configuration parameter.All sentences received by the GPS 25LP must be terminated with , but donot require the checksum *hh. The checksum is used for parity checking data and it isrecommended that the checksum be used in environments containing highelectromagnetic noise. It is generally not required in normal PC environments.Sentences may be truncated by after any data field and valid fields up tothat point will be acted on by the GPS 25LP.4.1.1 Almanac Information (ALM)$GPALM,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,<10>,<11>,<12>,<13>,<14>,<15>*hhThe $GPALM sentence can be used to initialize the sensor board's stored almanacinformation if battery back-up has failed.